1. Field of the Invention
The present invention relates to a vertical cavity surface emitting laser. Further, the present invention relates to an image forming apparatus using the vertical cavity surface emitting laser.
2. Description of the Related Art
A vertical cavity surface emitting laser (hereinafter, referred to as VCSEL) is a laser device for emitting a laser beam in a direction perpendicular to an in-plane direction of a substrate.
The VCSEL is advantageous in that the manufacturing process can be simplified, and a two-dimensional array is easily achieved as compared with an edge-emitting laser. In recent years, the VCSEL has been intensively studied.
A distributed Bragg reflector (hereinafter, referred to as DBR) is normally used as a reflective layer of the VCSEL.
The DBR is generally formed by alternately laminating a high refractive index layer and a low refractive index layer in an optical film thickness of λ/4. That is, the DBR is formed of a laminated structure in which the low refractive index layer and the high refractive index layer are alternately laminated. The laminated structure has a structure in which the refractive index is periodically changed in the thickness direction. Because of this periodic change of refractive index, an incident light to the DBR is reflected at the interfaces of the respective layers in phase with each other. As a result, the reflected waves on the respective layers are reinforced with each other, and the DBR functions as a reflector.
M. Grabherr et al., IEEE Photonics Technology Letters, vol. 9, no. 10, 1997, pp 1304 (Non-Patent Document 1) discloses a surface emitting laser that includes an oxidized structure on a layer that constitutes the DBR, and confines a current that is injected into an active layer to limit an emitting region of the active layer. Hereinafter, the surface emitting laser (also called “surface emitting laser of selective oxidation type”) having an oxidized confinement structure will be described with reference to FIG. 8.
Referring to FIG. 8, a DBR 802 including an Al0.2Ga0.8As high refractive index layer 808 and an AlAs low refractive index layer 809 being alternately laminated is disposed on a GaAs semiconductor substrate 801. An AlGaAs cavity 803 having three GaAs quantum wells and spacer layers is disposed on the DBR 802. A DBR 804 including an Al0.2Ga0.8As high refractive index layer 810 and an Al0.9Ga0.1As low refractive index layer 811 being alternately laminated and doped with a p-type impurity is formed on the cavity 803.
An AlAs layer 805 that is 30 nm thick is introduced into the low refractive index layer that is positioned at a portion closest to the cavity 803 among the Al0.9Ga0.1As low refractive index layers 811 that constitute the DBR 804. Further, an upper electrode 806 and a lower electrode 807 for current injection are formed on the upper portions of the DBR 804 and a rear surface of the semiconductor substrate 801, respectively. The DBR 802 is doped so as to be an n-type and the DBR 804 is doped so as to be a p-type.
The AlAs layer 805 is selectively oxidized in order to form the oxidized confinement structure. Because the AlAs layer 805 has a higher content of Al than other layers that constitute the DBR 804, its oxidation rate is higher, so that it is selectively oxidized in a high-temperature steam atmosphere. A semiconductor having a refractive index of about 3.1 changes into an insulator (aluminum oxide) having a refractive index of about 1.6 through this oxidation.
In Non-Patent Document 1, the selectively oxidized layer containing a larger amount of Al is disposed in the low refractive index layer 811 that constitutes the DBR as described with reference to FIG. 8. Then, the AlAs layer 805 becomes an oxidized region 812 and a non-oxidized region 813 through an oxidization process from the lateral direction to form the oxidized confinement structure.
Incidentally, the beam of a fundamental transverse mode of the vertical cavity surface emitting laser is a narrow-spread beam having a Gaussian intensity distribution whose center intensity is high. For that reason, as a light source for exposure in an electrophotographic device that demands a small-spot beam having a light intensity concentrated into the center, the beam of the fundamental transverse mode is more desirable than the beam of higher-order transverse modes.
Further, when multiple transverse modes lase at the same time, the transverse modes get unstable according to the operating conditions and the operating environments of the vertical cavity surface emitting laser. That is, hopping occurs between the modes, and the beam divergence angle and the intensity distribution change.
From the viewpoint of the above background, lasing of a stable single fundamental transverse mode is desirable.
Under the above circumstances, in Non-Patent Document 1, the diameter of a non-oxidized region (region 813 of FIG. 8) (hereinafter referred to as “confinement diameter”) is narrowed to provide the single fundamental transverse mode.
However, the inventors of the present invention have found that as a result of calculating the reflectance of the DBR including the non-oxidized region 813 and the reflectance of the DBR including the oxidized region 812 in the configuration illustrated in FIG. 8, the reflectance of the DBR including the oxidized region 812 is higher. (The details of the calculation conditions will be described later.)
That is, the inventors of the present invention have found that the provision of the oxidized confinement structure makes the reflectance of a peripheral portion of the DBR higher than that of a center portion thereof in the configuration of the conventional art. As a result, the reflectance of the light of a higher order mode having a larger amount of power distribution in the peripheral portion is higher than the reflectance of the light of a fundamental mode having a larger amount of power distribution in the center portion, thereby reaching a recognition for the first time that the conventional configuration has an influence of facilitating the lasing of the higher order mode.
In particular, when the confinement diameter of the oxidized confinement structure is expanded for the purpose of providing higher output, the transverse mode control with the oxidized confinement structure is insufficient, and the laser structure is affected by the DBR and liable to lase in multi-mode.